We harden off plants because those that are started indoors are not yet ready to be exposed to wind or sun. It is much like it is with humans, we need to slowly increase the amount of sunshine our skin is exposed to or we’ll get burned!
How to Harden Plants The best way to harden plants is to put them outside for a few hours the first day and then bring them in at night. Each day increase the number of hours they are exposed to sunshine until finally after 3 or 4 days you can leave them out all night. Partly cloudy days are perfect for hardening plants. If it’s a strong sunny day, set them in the shade, or ideally in part shade, for the first day. Using Reemay cloth, or floating row cover, over top of the plants to make shade is also a good approach to hardening plants on a sunny day.
Try this trick for a little extra protection: Cut out the bottom of a gallon milk jug and place over your new transplant, burying the bottom of the jug in to the soil an inch or so deep. Remember to remove the jugs when you water the plants, or water each one through the whole on top of the jug.
What Happens If I don’t Harden My Plants? Tender plants will get burned by the sun, the shock of cold, or by the wind. Some plants may recover, even fully, from burn but their growth will be set back a few weeks while they recover.